This invention relates to bidirectional transport of sprocketed film and, more particularly, to a method and apparatus for contact film printing which meets high quality standards.
Contact film printers are employed to expose a roll of sprocketed raw film to the frames of photographic images on a roll of sprocketed master film while the master film and raw film are transported continuously together in contact.
In a bidirectional continuous contact printer, the sprocket holes of the raw film and master film engage teeth in a main sprocket wheel located at the exposure aperture. To maintain precise control over the position of the sprocket holes of the master film and raw film during the exposure process, the raw film and master film pass over tensioning means, such as an auxiliary sprocket wheel and a tensioning arm, before reaching the sprocket wheel. The tensioning means draw the leading edges of the sprocket holes in the films that engage the teeth in the main sprocket wheel against such teeth, thereby precisely aligning the sprocket holes of the raw film and the master film during exposure. As a result, the main sprocket wheel pulls the films past the exposure aperture as it rotates. The edges of each tooth of the sprocket wheel against which the leading edges of the sprocket holes are drawn must be polished to provide a smooth, even surface.
CONVENTIONALLY, IN A BIDIRECTIONAL CONTINUOUS CONTACT PRINTER, TWO TENSIONING MEANS ARE PROVIDED SO, THAT FOR BOTH DIRECTIONS OF FILM TRANSPORT, THE LEADING EDGES OF THE SPROCKET HOLES OF THE FILMS THAT ENGAGE THE TEETH OF THE MAIN SPROCKET WHEEL ARE DRAWN AGAINST SUCH TEETH, AND THE MAIN SPROCKET WHEEL PULLS THE FILMS PAST THE EXPOSURE APERTURE AS IT ROTATES IN BOTH DIRECTIONS. Both edges of each tooth must be polished to provide a smooth even surface.